Page:Popular Science Monthly Volume 10.djvu/274

260 The presence of graphite, or plumbago, in the Eozoic rocks is by many regarded as a still stronger argument for the former existence of vegetation. As graphite is nearly pure carbon, it is easy to believe that it has accumulated from the remains of plants. Greater changes have been effected in its mass through metamorphism than in the alterations of the ore-beds. No traces of vegetable structure have yet been detected in graphite, so that no evidence as to the nature of the earliest plants can be afforded from morphology.

—What species of vegetation can we imagine to have existed in these early periods? Possibly we may derive a hint as to its nature from the general course of plant-development in later ages, and assume that there has been a correspondence between the order in which the different classes have appeared and their successive stages of complexity of structure. The simpler forms should appear first; or, reversing the statement, if we find a succession of all the higher forms of growth in later times, it is reasonable to expect in the still earlier periods larger developments of the inferior cryptogams, such as now play a comparatively insignificant part in the economy of Nature. Their easy decomposition would prevent the preservation of their specific shapes as fossils.

To particularize, we have among the lower orders of terrestrial vegetation the lichens, by some thought to be the parent of the fungi and algæ, since they can be resolved into two different plants, a fungus parasitic upon an alga; the mushrooms, puff-balls, mildews, blight, or fungi; the hepaticæ, and mosses. Of aquatic vegetation there are the numerous protophytes, the diatoms, with their siliceous shells; the desmids, the coccoliths, with their lenticular calcareous disks; the nullipores and corallines, making calcareous incrustations; and the great family of Algæ, simple, branched, and confluent. These afford us abundant material from which we may reconstruct the Eozoic meadows, forests, and submarine carpets.

The present system of plants seems to have originated in the Cretaceous period. The older Mesozoic gives us the cycads and tree ferns, like those of the Asiatic tropics. The Paleozoic formations furnish a unique assemblage of combined cryptogamous and phenogamic nature of types not now existing. Granting that the two divisions of the plant kingdom are of equal importance in the line of development, we find ourselves in Silurian times only half-way back to the starting-point. If the Cambrian should furnish us with representations of the mosses and lichens, we might expect in Eozoic times some of these, together with the protophytes, etc., in order to complete the systematic and orderly development of the plant kingdom in time. Furthermore, the later ages have afforded gigantic representations of the higher orders. Why, then, should not the Eozoic land have had its forests of mushrooms and arborescent lichens; its swamps of diatoms, confervæ, the charæ and desmids,